Bifocal lens



Feb. 21, 192s. 1,659,966

E. D. TlLLYER BIFOCAL LENS Filed April 12. 1926 attorney;

Patented Feb. 21, 1928.

UNITED STATES PATENT OFFICE.

EDGAR D. TILLYEB, F SOUTHBRIDGE, MASSACHUSETTS, ASSIGNOB T0 AMERICAN OPTICAL COMPANY, 0F SOUTHBBIIDGE, MASSACHUSETTS, A VOLUNTARY ASSOCIA TION OF MASSACHUSETTS.

BIFOCAL LENS.

This invention relates to bifocal lenses and has more particular reference to a fused bifocal lens wherein provision is made to n eutralize the astigmatism through the reading ,portion of the major lens as well as through the minor lens.

The rincipal object of this invention is to provide means by which the astigmatism inherent in the major blank of a fusedbi lowing specitication taken in connection withv the accompanying drawings, and it will be understood that I may make any modifications in the construction and arrangement of parts without departing from the spirit of the invention as set forth in the appended claims, and I, therefore, do not wish to be limited to the exact arrangements shown. The preferred form only of the invention has been shown by way of illustration.

Referring to the drawings in which similar characters of reference are employed to denote corresponding parts throughout:

1 is a front view of a pair of spectacles embodying the invention;

Figure 2 is a cross section on line 2-2 of Fi re 3;

igure 3 is a front or plan view of a bifocal lens embodying the invention.

In making my bifocal lens I provide a major blank 1 of one index of refraction which I rovide with the surfaces 2 and 3 to provi e the best shape for desired c orrection of oblique 'pencils of rays coming from a distant object indicated at 4 in Figure 2. It will be seen that this is not the best shape for pencilsof rays from near by readin objects as shown at 5, Figure "2, ybecause t ese rays converge to a point 6. Itis, therefore, clear that these rays `or pencils will ire a diferent shape lens from -that reqiiilredv for the distance portion, and that as the surfaces 2 and 3 have been put on the lens there will be an astigmatism throu h the reading ortion of the lens due to t e shape of the istance blank 1.

I next face 3 of the 4major blank 1 and :fuseor grind a countersink 7. in the surotherwise secure in the countersink 7 the minor lens 8 of a different index of refraction from that of the major lens 1. The surface 7 ofthe countersink if spherical will also have an inherent astigmatism due to the oblique vision which vis had through the reading portion, as indicated b the line 9, Figure 2. Therefore, it will e seen that through the reading portion of the composite blanks 1 and 8 I have two astigmatic errors, one due to the shape of the curves on the blank 1 and the other due to the countersunk segment 8. It is the object of my invention to neutralize these two astigmatisms and it may be done in any one of three different ways. The composite astigmatism, of course, will be found on some particular meridian through the reading portion. Now, I can compensate for this compound astigmatism by grinding the countersink 7 to a toric shape instead of to aspherical shape having the excess astigmatism on the desired meridian to neutralize the inherent astigmatism of the segment as well as of theshape of the major blank, or I may make my countersink 7 a. spherical curve and fuse the segment 8 t erein and then bend the lens over a torio block while the lens is heated to a int of plasticity so that the eountersin v7 is stretched in bending to a toric form having one ymeridian changed to an amount to'compensate the composite astigmatism heretoore referred to, or I may grind the surface 3 over my major and minor blanks 1 and 8 to a toric shape having the excess in one rmeridian suicient to compensate the composite astigmatism aforesaid and grind on the other face 2 of thev lens another toric curve which shall be so positioned that its fexcess in one meridian will compensate the -excess that has been placed on the other side -ground and polished on the usual prior art lens making machines, as likewise Iare the i i surfaces of the eountersink 7 and the button 8 fused in the countersink 7 in the usual way. One surface .3 is continuous over both the distance and the reading portions of the lens as is usual with the so-called bifocas.

It is clear that I may locate the segment 8 in an desired position with respect to the ma'or lank 1.

rom the fore oing it will be apparent that I have provi ed simple, efficient and inex ensive means of making an o hthalmic bi ocal lens havin the best con ition for distance vision an having` the various astigmatisms through'the reading portion com pensate'd and corrected.

Having described my invention, I claim:

1. An o hthalmic lens having a major lens portion with lens surfaces on the front and rear sides thereof to produce the required prescri tive power for distant vision, a minor ens portion overlyinfg a apart of said major lens portion and sur ace to produce the combination with the major lens portion over which it lies the required prescriptive power for reading vision, said overlying surfaces of the major and minor lens portions havin a composite variation in one meridian-su stantially equal and opposite to the composite astgmatlc aberration in said meridian of the overlying major and minor portions of a lens of the same prescriptive power withunvaried su'rfaces whereby the said'astigmatic aberration is substantially neutralized. A

2. The process of making an o hthalmic lens having overlying distance an fields free from astigmatic aberration in the overlying fields comprising computing the front and rear surfaces of said lens to give l0 the required prescriptive powers of distance and reading vision, computing the composite astigmatic aberration and the meridian thereof through the overlying elds, and surfacing a lens blank with front and rear optical surfaces p to give the calculated prescriptive distance and reading` owers with a composite variation through t e overlying fields substantially equal and opposite to the calculated astigmatic aberration in the said meridian to substantially neutralize said composite astigmatic aberration of the overlying fields.

' .EDGAR D. THJLYISR.A

reading v 

